To achieve smooth and uniform surface ablation with a pulsed laser beam of high repetition rate and small spot size, a fast and smooth scanning and a proper scanning pattern are crucial. When an intense UV laser pulse impinges a corneal surface, for instance, a plume of decomposed tissue is ejected from the surface. This ejected material may affect the energy disposition of the next pulse. Besides, the stress and heat generated from the ablation process may build up if the pulsed laser beam is not scanned fast enough. Each pulse creates an ablated pit having a typical depth of a fraction of a micron. A uniform ablation profile can be expected only when these pits are arranged in a proper disposition pattern.
There are some 500 U.S. patents associated to scanning a laser beam. The present invention relates specifically to scan a pulsed laser beam for surface ablation. In particular, the present invention relates to scan a pulsed laser beam with a high repetition rate (about a kilohertz) and a small spot size (a fraction of a millimeter) for smooth and uniform surface ablation. A direct application of the present invention is to scan a pulsed laser beam for photo-refractive surgery on a cornea to correct vision disorders.
A few scanning methods have been proposed for photo-refractive surgery. In U.S. Pat. Nos. 4,665,913 and 4,718,418, L'Esperance Jr. presented a method to scan laser pulses with a uniform power over a squared cross section. The scanner is synchronized with the pulses to achieve precise disposition of the pulses. Lin demonstrated in U.S. Pat. No. 5,520,679 a method to achieve smooth ablation by accurately controlling the beam spot size and carefully overlapping the pulses in a single layer. A 100 Hz-pulsed UV laser beam was scanned linearly to show a desirable result. Simon and Wuang disclosed in U.S. Pat. No. 5,599,340 a method of disposing the laser pulses over a programmed pattern in a random process. For a pulsed laser of low repetition rate, this programmed random process can generate a similar result as that of continuous scanning.
When the pulse repetition rate is increased to the kilohertz level, however, the above mentioned scanning methods become non-practical. At a kilohertz repetition rate, the time interval between the pulses is only a millisecond. This is too fast for today's scanner to synchronize precisely the scanner mirror position with the laser pulses. Uniform disposition of the pulses becomes impossible with linear scanning because of the slowdown of the scanning when the beam turns around. Close overlapping between the pulses is not desirable because the plume from the previous pulse will affect the energy disposition of the next pulse. Besides, an accurate spot size on the target is practically impossible to defined and to maintain when the pulse energy varies with time.